You need some prefix, but it doesn't have to be c:, that's just the convention. The prefix is specified when you declare the tag library at the top of the JSP, and it can be anything you like (within reason). The important part is the namespace URI (also specified in the JSP header), and that has to be a specific value (http://java.sun.com/jsp/jstl/core in this case).

But you must have a prefix of some form, otherwise the JSP engine won't recognise it.